How do I open an obj file?
Only .blend files can be opened by Blender. Other file formats can be only imported.

Obj Import

Go to File ‣ Import ‣ Wavefront (.obj) and choose the file.

Remember to save the Blender file as a .blend file. If you wish to export the obj file go to the menu item File ‣ Export ‣ Wavefront (.obj).